1. Field of the Invention
The present invention relates to a memory circuit in a signal processing unit, in which the logic state is not erased after the power is turned off.
2. Description of the Related Art
Signal processing units such as central processing units (CPUs) vary in structure depending on the intended use. A signal processing unit generally has a main memory for storing data or program and other memory circuits such as a register and a cache memory. A register has a function of temporarily holding a data signal for carrying out arithmetic processing, holding a program execution state, or the like. Meanwhile, a cache memory, which is located between an arithmetic unit and a main memory, is provided to reduce low-speed access to the main memory and speed up the arithmetic processing.
In a memory circuit in a signal processing unit, such as a register or a cache memory, input of a data signal needs to be performed at higher speed than in a main memory. For this reason, in general, a flip-flop or the like is used as a register, and a static random access memory (SRAM) or the like is used as a cache memory. In other words, such a register, a cache memory, or the like is a volatile memory circuit which loses a data signal after the supply of power is stopped.
In order to achieve low power consumption, a method in which the supply of power to a signal processing unit is temporarily stopped while input/output of data signal is not conducted has been suggested (see Patent Document 1, for example). In the method in Patent Document 1, a nonvolatile memory circuit is located in the periphery of a volatile memory circuit such as a register or a cache memory, and the data is temporarily stored in the nonvolatile memory circuit. Thus, the register, the cache memory, or the like in the signal processing unit stores a data signal even while the supply of power is stopped.
In the case where the supply of power to a signal processing unit is stopped for a long time, a data signal in a volatile memory circuit is transferred to an external memory circuit such as a hard disk or a flash memory before the supply of power is stopped, so that the data signal can be prevented from being lost.